In the optical subscriber network system, a passive optical network (PON) system that is standardized in IEEE or ITU-T is widely adopted. The PON system is configured such that optical network units (ONU) that are arranged respectively to a plurality of subscriber devices are connected to an optical line terminal (OLT) arranged in the accommodation station through optical splitting device and one optical fiber arranged inside and outside the accommodation station.
In the PON system, an up-signal and a down-signal are transmitted bi-directionally on the same optical fiber by different wavelengths. The down-signal is a successive signal formed by multiplexing signals that are output from the OLT by using a time division multiplexing (TDM) technology. The ONU located in the subscriber device takes out a time slot signal required for the ONU itself from the successive signals split in the optical splitting device. In addition, the up-signals are burst signals that are intermittently transmitted from the ONU, and are coupled in the optical splitting device to produce a TDM signal, which is sent to the accommodation station.
Since the up-signal varies in intensity depending on a difference in distance from the subscriber device to the optical splitting device or an individual difference of transmitter output in the ONU, a wide input dynamic range is required for the receiver. In the PON system, the OLT and a few pieces of the optical fibers arranged in the accommodation station can be commonly used among a plurality of subscriber devices by using the TDM technology. Therefore it is possible to economically provide an optical access service at a high speed exceeding gigabits.
However, the PON system has been already put into practice, but, expansion of an allowance transmission loss budget is still demanded. When the expansion of the loss budget can be realized, an accommodation efficiency of the PON system can be improved “by increasing the split number to increase the number of the subscriber devices that one OLT accommodates” or “by lengthening the optical fiber transmission path to expand the accommodation area”.
For realizing the expansion of the loss budget, there is widely considered a method in which an two-way optical amplifier that individually amplifies the up/down signal is used to compensate for losses of the multi-splitting optical splitting device or the lengthened optical fiber transmission path. In many cases, however, it is assumed that the two-way optical amplifier is arranged in the optical fiber transmission path to be used as a relay. Considering the easiness of ensuring a power source for driving the two-way optical amplifier, it is desirable that the two-way optical amplifier is arranged in the accommodation station and the optical amplifier for amplifying the up/down signal is used as a front-side optical amplifier and a rear-side optical amplifier respectively.
In a case of arranging the front-side optical amplifier in the accommodation station for use, however, when a strenuous burst optical signal is incident to the front-side optical amplifier, there occurs a problem that an optical signal exceeding an input dynamic range reaches the receiver in the accommodation station, the up-signal cannot be normally received.
On the other hand, NPL 1 proposes a method in which coupling losses of the up-signal in the optical splitting device are reduced, thereby obtaining the effect similar to a case of equivalently expanding the loss budget by the reduced loss amount.
FIG. 1 shows the configuration of a PON system 100 shown in NPL 1. In FIG. 1, there is shown the PON system 100 in which an ONU 111 that is arranged in each of subscriber devices 110 is connected to an OLT 136 in an accommodation station 130 through an optical splitting device 120. The accommodation station 130 is provided with wavelength multiplexing/demultiplexing devices 131, an optical splitting device 132, an optical coupling device 133, and the OLT 136 including a transmitter 134 and a receiver 135. It should be noted that in FIG. 1, the OLT 136 is formed of the transmitter 134 and the receiver 135, but may include the wavelength multiplexing/demultiplexing device, the optical splitting device and the optical coupling device.
In the PON system 100 exemplified in FIG. 1, in addition to split an optical signal by the optical splitting device 120 arranged outside of the accommodation station 130, the optical signal is split also inside of the accommodation station 130 (a case of four splits is shown in FIG. 4). Further, in the PON system 100 exemplified in FIG. 1, one optical splitting device 132, one optical coupling device 133, and wavelength multiplexing/demultiplexing devices 131 in number equal to the split number inside of the accommodation station are used to perform the splitting of the down-signal and the coupling of the up-signal inside of the accommodation station 130 simultaneously.
In the PON system 100, the down-signal transmitted from the transmitter 134 is split by the optical splitting device 132, and the split down-signals are sent to the wavelength multiplexing/demultiplexing devices 131 respectively. The optical splitting device 132, as described above, can perform the coupling of the up-signals at the same time with the splitting of the down-signal, but in the PON system 100, only the splitting of the down-signal is performed. On the other hand, the up-signal of each split inside of the station is multiplexed/demultiplexed from the down-signal in the wavelength multiplexing/demultiplexing device 131. Further, the multiplexed/demultiplexed up-signals are multiplexed in the optical coupling device called a mode coupler, which thereafter, is received in the receiver.
FIG. 2A and FIG. 2B are configuration examples of the optical coupling devices. FIG. 2A shows the configuration example of the optical coupling device using a planer lightwave circuit (PLC), and FIG. 2B is the configuration example of the optical coupling device using a fusion single mode fiber (SMF) waveguide. FIG. 2A shows a conventional optical coupling device 200 provided with a multi-mode fiber (MMF) 210 and a PLC 220. FIG. 2B shows a conventional optical coupling device 200 provided with the MMF 210 and a fusion SMF waveguide 230. The MMF 210 has a clad 211 and a core 212, and an up-signal is propagated through the core 212 for transmission.
The PLC 220 shown in FIG. 2A includes a slab waveguide 221, and a SM waveguide 222 connected to the slab waveguide 221. The fusion SMF waveguide 230 shown in FIG. 2B includes a fusion portion 231 and a SMF 232 fusion-connected to the fusion portion 231. Any of the optical coupling devices shown in FIG. 2A and FIG. 2B has the configuration of bundling the plural SM waveguides 222 or one-end surfaces of the SMFs 232, which are radially arranged on a plane. The coupling of optical signals in the optical coupling device is performed in each of the slab waveguide 221 and the fusion portion 231 shown in FIG. 2A and FIG. 2B.
Each of the optical coupling devices shown in FIG. 2A and FIG. 2B keeps the confinement of optical signals incident from the other end surface of the SM waveguides 222 or the SMFs 232 in a planar vertical direction, and at the same time, eliminates the boundary between the respective SM waveguides 222 or between the respective SMFs 232. In this state, the optical signals are coupled after proceeding by a predetermined distance. Since a mode field diameter of the coupled optical signal is wider than a mode field diameter of the SM waveguide 222 or the SMF 232, the MMF 210 is connected to the coupling portion of the optical signals, and the coupled optical signal is confined in the optical fiber without leakage. Consequently, it is possible to overcome the lacking of the optical splitting device that the coupling loss is in principle increased according to an increase in the split number (1/N in N×1 splits).
In fact, as shown in NPL 2, in a case of the optical coupling device of 8×1 by the configuration shown in FIG. 2A, although the principle loss is 9 dB in a conventional splitter circuit by a combination of Y splitting elements as in the case of the slab waveguide, the coupling loss within 2 dB is realized. In addition, as shown in NPL 3, in a case of the optical coupling device of 16×1, the principle loss is 12 dB, but an improvement of the principle loss equal to or more than 7 dB, that is, the worst coupling loss is equal to or less than 5 dB, is realized.